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1623. O horizon : Is an organic horizon formed or forming from an accumulation of organic material deposited on the surface, that is not saturated with water for more than a few days a year and contains 20% or more organic carbon. O horizons are the organic horizons that develop on top of some mineral soils. For example, the raw humus which covers certain acid soils .The organic material in O horizons is generally decomposed and occurs under naturally well-drained conditions. The designation does not include horizons formed by a decomposing root mat below the surface of the mineral soil which is characteristic of A horizons. O horizons may be buried below the surface.